Formation of thermotropic mesophase

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

In this chapter, the structural properties of thermotropic and lyotropic liquid crystals will be compared. In a first step, the driving forces for the formation of the mesophases, as well as the building blocks of the two types of liquid crystals will be analyzed. Afterwards, the structures and properties of the most important liquid crystalline phases will be described, as far as they are important in the context of this thesis. 

Even though lyotropic and thermotropic liquid crystals share the same state of matter, the driving forces for the formation of the mesophases differ substantially. To understand this, the molecules which form the respective liquid crystalline phases have to be examined in more detail. Figure 3.1 shows typical examples of such molecules. 

In thermotropic liquid crystals, the formation of particular mesophases depends mainly on the temperature. On cooling, the structure of the mesophases becomes more and more ordered and thus less symmetric. For thermotropic mesophases formed by calamitic mesogens a fixed sequence was found [37, 38] ... 

In dependence on how the liquid crystalline phase is formed, lyotropic and thermotropic mesophases can be distinguished. Thermotropic mesophases are induced by temperature and require an anisometric shape of the mesogen. In contrast, amphiphilic molecules (e.g. surfactants) are able to form lyotropic mesophases in the presence of suitable solvents. The formation of lyotropic mesophases depends on the concentration of the mesogen as well as on the temperature. Furthermore, certain molecules may form both thermotropic and lyotropic mesophases (e.g. phospholipids ). 

The monofunctional P-U complex (supermolecule 19, Table 3) has been shown to form disklike dimers that can self-assemble into columns displaying thermotropic behavior. Each disk has a thickness/diameter ratio of 0.1 [155], The formation of columnar mesophases by similar discotic supermolecules is described in the previous section when cases in which liquid crystallinity and growth are hierarchically related, coupled, or totally uncoupled were considered. Lack of data on the equilibrium constants, or DP, prevents a definite assessment of the assembling mechanism of discotics based on supermolecule 19. 

In the formation of carbonaceous mesophase by thermolysis (pyrolysis) of isotropic molten pitch, the development of a liquid-crystalline phase is accompanied by simultaneous aromatic polymerization reactions. The reactivity o/pitch with increasing heat treatment temperature and its thermosetting nature are responsible for the lack of a true reversible thermotropic phase transition for the bulk mesophase in most pitches. Due to its glass-like nature most of the liquid-crystalline characteristics are retained in the super-cooled solid state. 

Peripheral modification of the receptor molecules may allow the existence of stacking structures in soft materials such as thermotropic liquid crystals. Differential scanning calorimetry (DSC) analysis of the octane xerogel of Ic Cl -TATA+ suggested the formation of a mesophase as observed in the phase transitions at 88 and 42°C upon first cooling from the isotropic liquid state (Iso) and at 44 and 96°C upon second heating. 

Figure 3. Schematic representation of thermotropic mesophases based on charge-transfer induced formation of intercalated donor-acceptor stacks Ncoi nematic columnar and Colh columnar hexagonal ordered mesophase (see footnote 1 in Sec. 3 commenting on the nomenclature of such columnar mesophases). electron donor, electron acceptor.

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Meier et al. have also contributed to the field of cyclic PAV oligomers with the synthesis of cyclic dl -trans (all- ) trimers (e.g. 113) containing alkoxy-substituted 1,7-naphthylene and 1,9-phenanthrylene building blocks, via a Siegrist-type trimerizing olefination [134,135]. Suitable substitution at the periphery of the cyclic trimers allows for the formation of stable, thermotropic discotic mesophases [134]. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

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